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Branton Campbell in the lab

Branton J. Campbell

Professor

Department of Physics & Astronomy

Brigham Young University

Provo, UT 84602, USA

Tel: 801-422-5758, Fax: 801-422-0553

Email: branton_campbell[at]byu.edu

//physics.byu.edu/faculty/campbell/

Research Interests

I apply state-of-the-art x-ray and neutron scattering techniques to study local and long-range structures in a variety of complex solids, including fast-ion conductors, ferroelectric relaxors, high-temperature superconductors, and colossal magnetoresistive manganites, where nanoscale structural features influence macroscopic physical properties. This includes the development of symmetry-mode analysis as a tool for the determination, refinement and interpretation of distorted structures involving lattice strains, atomic displacements, magnetic moments and occupational orderings at both commensurate and incommensurate wavevectors.

Selected Publications

A general algorithm for generating isotropy subgroups in superspace

By Harold T. Stokes and Branton J. Campbell

Abstract: This paper presents a general algorithm for generating the isotropy subgroups of superspace extensions of crystallographic space groups involving arbitrary superpositions of multi-k order parameters from incommensurate and commensurate k vectors. Several examples are presented in detail in order to illuminate each step of the algorithm. The practical outcome is that one can now start with any commensurate parent crystal structure and generate a structure model for any conceivable incommensurate modulation of that parent, fully parameterized in terms of order parameters of irreducible representations at the relevant wavevectors. The resulting modulated structures have (3 + d)-dimensional superspace-group symmetry. Because incommensurate structures are now commonly encountered in the context of many scientifically and technologically important functional materials, the opportunity to apply the powerful methods of group representation theory to this broader class of structural distortions is very timely.

ISOSUBGROUP: an internet tool for generating isotropy subgroups of crystallographic space groups

By Harold T. Stokes, Seth van Orden, and Branton J. Campbell

Abstract: ISOSUBGROUP, the newest member of the ISOTROPY Software Suite (http://iso.byu.edu), generates isotropy subgroups of crystallographic space groups based on superpositions of multiple irreducible representations, along with a wealth of information about each one. Like the original ISOTROPY program, its scope is general rather than being restricted to common types of order parameters of a user-specified parent structure. But like the newer ISODISTORT program, its user-friendly interface has menu-driven selections. This combination of features has been oft requested but unavailable until now. Program output includes information about the subgroup symmetry, ferroic species, phase-transition continuity, active k vectors, domains and secondary order parameters.

An Exhaustive Symmetry Approach to Structure Determination: Phase Transitions in Bi₂Sn₂O₇

By Harold T. Stokes and Branton J Campbell (et al.)

Abstract: The exploitable properties of many materials are intimately linked to symmetry-lowering structural phase transitions. We present an automated and exhaustive symmetry-mode method for systematically exploring and solving such structures which will be widely applicable to a range of functional materials. We exemplify the method with an investigation of the Bi₂Sn₂O₇ pyrochlore, which has been shown to undergo transitions from a parent γ- cubic phase to β- and α- structures on cooling. The results include the first reliable structural model for β-Bi₂Sn₂O₇ (orthorhombic Aba2, a = 7.571833(8), b = 21.41262(2), c = 15.132459(14)) and a much simpler description of α-Bi₂Sn₂O₇ (monoclinic Cc, a = 13.15493(6), b = 7.54118(4), c = 15.07672(7), β = 125.0120(3)) than has been presented previously. We use the symmetry-mode basis to describe the phase transition in terms of coupled rotations of the Bi₂O’ anti-cristobalite framework which allow Bi atoms to adopt low-symmetry coordination environments favoured by lone pair cations.

Structural state and magnetic properties of multilayer-graphene/Fe composites

By B. J. Campbell (et al.)

Abstract: Results are given of measurements of X-ray and neutron diffraction, small-angle neutron scattering, and field dependences of the magnetization for nanocomposites formed by multilayer graphene and iron particles. Four samples have been investigated that differed in the content of Fe as follows: 0.75, 30, 45, and 70 wt %. The calculations of the X-ray diffraction and neutron diffraction patterns show that the size of graphene particles is about 4 nm, and the size of the iron domains of coherent scattering is no less than 50 nm. The content of the Fe particles in the composites has been estimated based on the data of the measurements of neutron diffraction, small-angle neutron scattering, and magnetization. The values of the iron concentration obtained by different methods differ quite significantly. The difference is caused by the specific character of the methods of measurements.

La-Dopant Location in La-Doped γ-Al₂O₃ Nanoparticles Synthesized Using a Novel One-Pot Process

By Stacey J. Smith, Baiyu Huang, Calvin H. Bartholomew, Branton J Campbell, Juliana Boerio-Goates, and Brian F. Woodfield

Abstract: We have recently developed a ‘solvent-deficient’ method of synthesizing high surface area γ-Al2O3 nanoparticles that show promise for catalyst support applications. Here, we investigate doping the alumina with La3+ to stabilize the gamma phase to higher temperatures. The 1-pot method for synthesizing La-doped γ-Al2O3 nanoparticles developed here has several advantages over conventional methods including requiring only 3 hours instead of 3 days and wasting no lanthanum. TEM, X-ray PDF, and BET analyses as a function of calcination temperature show that the La stabilizes the gamma phase by 100°C. In order to begin understanding the mechanism of stabilization, the location of the La3+ atoms in our doped γ-Al2O3 nanoparticles is investigated via X-ray PDF and EXAFS analyses which indicate that the La dopant adsorbs as single, isolated atoms on the γ-Al2O3 surface. As calcination temperature increases, the immediate oxygen coordination shell of the La becomes increasingly like La2O3 though an extended La2O3 lattice is not formed due to the sparse (3 wt%) concentration of La atoms. During the γ-to-α transition, the La environment changes drastically to become more like LaAlO3 than La2O3, suggesting that the La is enveloped by the alumina lattice during the alpha phase transition though an extended LaAlO3 lattice is not formed.